Hi experts,

I have a '91 D250 auto. that keeps downshifting out off OD. I think the problem is with that little black box on top off the Injection pump.

My question is Does the speedometer sensor also control shifts?

Because my speedometer has a mind of its own sometimes. It will fulculate(sp?) on the speed between 0 and 100, then go back to working normal. Sometimes this action is the same time as it down shifts and sometimes no downshift, just fulculating.

I want to find out before I put a new black box on so it doesn't burn it up if they work together.

Thank you for any help.

AndyT

 

Yes, the vehicle speed sensor (VSS) also controls OD. If your speedometer is "funky" sometimes, I'd replace the VSS. I've never done that, but I've heard it's pretty easy and cheap.

If that isn't it, you can try the throttle position sensor (TPS-the black box on the injection pump). Sometimes it just needs adjustment, with which this should help http://www.piersdiesel.com/Tech1GTPS.htm . Otherwise you can replace the sensor, or replace it with a potentiometer, as seen here https://www.dieseltruckresource.com/...ad.php?t=42003

PM me if you need a TPS.

 

Also, this might seem very strange but I had the same problem. My O/D in my 1992 was acting weird too. Sometimes it would not work at all, sometimes it would work just fine, and other times it was intermitten. For me it turned out to be insulation on my crankcase wires wore off and was grounding against the block. For some reason, part of the overdrive AND charging systems are wired through the crank sensor. I replaced my crank sensor and both my overdrive and charging problems went away.

Hope this helps and good luck!

 

cranks sensor could be your next step after the VSS

the VSS is real easy to replace!!!!!

 

Thanks Guys where is the VSS? And how much do they cost? On the '93 I had years ago I replaced several TBS but never had the funky speedo.
Thanks
Andyt

 

The VSS is on the tailshaft housing of the trans where you would expect to find a speedo cable.

 

Where can you find a crank sensor? cummins?

 

Thanks Guys for all of the advice. I replaced the VSS yesterday, and checked the TPS. The TPS checked out good and now everything is working great. I don't think it has shifted that good since I bought it.

While I was under the hood I turned a few of those screws on the pump and that ol' truck came to life, WOW.

Anyways Thanks for the help.

Thankyou.
Andyt

 

Yes or the Dodge dealer, but it's probably more expencive!